1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece to be polished, and more particularly to a polishing apparatus for polishing a workpiece having a thin film formed thereon, such as a semiconductor wafer, to a flat mirror finish.
2. Description of the Related Art
As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and distances between these circuit interconnections become smaller. In case of photolithography which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because a depth of focus of an optical system is relatively small. A polishing apparatus for performing chemical mechanical polishing (CMP) has been used for planarizing semiconductor wafer.
This type of polishing apparatus comprises, as shown in FIG. 1, a polishing table 302 having a polishing cloth (polishing pad) 300 attached thereon and constituting a polishing surface, and a top ring 304 for holding a substrate W as a workpiece to be polished, such as a semiconductor wafer, in such a manner that a surface to be polished faces the polishing cloth 300. A semiconductor wafer W is polished by this polishing apparatus as follows: The polishing table 302 and the top ring 304 are independently rotated, and, while a polishing liquid is supplied 25 from a polishing liquid nozzle 306 provided above the polishing table 302, the semiconductor wafer W is pressed against the polishing cloth 300 on the polishing table 302 at a predetermined pressure by the top ring 304. For example, a suspension of fine polishing particles of silica or the like in an alkali solution is used as the polishing liquid supplied from the polishing liquid nozzle 306. Thus, the semiconductor wafer W is polished to a flat mirror finish by a combined effect of a chemical polishing effect attained by the alkali and a mechanical polishing effect attained by the polishing particles.
When the semiconductor wafer W is brought into contact with the polishing cloth 300, and the polishing table 302 is rotated to perform polishing, a polishing liquid or ground-off particles of semiconductor material are attached to the polishing cloth 300, resulting in a change in properties of the polishing cloth 300 and deterioration in polishing performance. Therefore, if an identical polishing cloth 300 is repeatedly used for polishing semiconductor wafers W, problems such as lowered polishing rate and uneven polishing are caused. In order to overcome such problems, a dresser 308 is provided in the polishing apparatus, and the polishing cloth 300 is dressed by the dresser 308 at a time of replacement of a semiconductor wafer W to be polished, for example. During a dressing process, while a dressing element attached to a lower surface of the dresser 308 is pressed against the polishing cloth 300 on the polishing table 302, the polishing table 302 and the dresser 308 are independently rotated to remove the polishing liquid and the ground-off particles of the semiconductor material which are attached to the polishing surface and to flatten and dress the polishing surface in its entirety, whereby the polishing surface is regenerated. This dressing process is also referred to as a conditioning process.
During the dressing process, a portion of the dressing element brought into sliding contact with the polishing surface may come off the lower surface of the dresser and remain on the polishing surface in some cases. If the portion of the dressing element that has come off the lower surface of the dresser remains on the polishing surface, then a surface of a subsequent semiconductor wafer to be polished may be scratched by this portion of the dressing element.
For example, in the case of a diamond dresser, which comprises a dressing element constituted by particles such as diamond particles electrodeposited on a lower surface of a dresser, in order to reduce a number of diamond particles which come off the dressing element, it has been attempted to reduce a number of suspended particles present on the lower surface of the dressing element by performing an initial run-in or positioning the diamond particles at increased intervals. However, it is highly difficult to completely eliminate diamond particles from coming off the dressing element.
After a semiconductor wafer is polished by the top ring, polishing liquid used during the polishing process and ground-off particles of semiconductor material may possibly remain on the polishing surface of the polishing cloth. Since these remaining polishing liquid and ground-off particles tend to scratch a surface of a semiconductor wafer, it is necessary to remove them before a subsequent polishing process is performed.